1. Field of the Invention
This invention relates to abrasion resistant coatings for polymeric substrates, especially polymeric ophthalmic devices, and, more particularly, to an amine hardened epoxy silane coating composition having a shelf life that has been extended by the addition of a metal-containing stabilizer.
2. Description of the Related Art
Polymeric (i.e., plastic) materials are increasingly being used to replace glass because of their low weight and impact resistance. However, plastics are softer than glass and scratch and abrade more easily. This is particularly problematic when plastics are used for ophthalmic devices such as eyeglass lenses where minor scratches or abrasions may greatly impair their function. Consequently, considerable efforts have been devoted to developing scratch and abrasion resistant coatings for plastic substrates.
For example, U.S. Pat. No. 3,961,977, "Coating Compositions and Process for Producing Coated Articles," issued Jun. 8, 1976 to H. Koda et al., discloses a transparent, scratch resistant coating comprising a partially hydrolyzed carbon-functional silane derived from an aminoalkylalkoxysilane having primary and/or secondary amine groups and an epoxyalkylalkoxysilane. Reportedly, the improved abrasion resistance results from a cross-linking reaction between the epoxy groups and the amine groups. Upon mixing, the two materials yielded a solution that reportedly remained stable at 50.degree. C. for more than thirty days.
U.S. Pat. No. 4,378,250, "Organosilicone Coating Compositions," issued Mar. 29, 1983, to G. Treadway et al., discusses the previous patent and states that the "stability of the coating solutions disclosed in U.S. Pat. No. 3,961,977 is achieved by high dilution, which is undesirable in that necessary (desirable) film thickness can only be achieved by the application of several coats. Also, compositions as taught by this patent are believed to be inferior in abrasion resistance and have been criticized in the literature as having poor resistance to weathering, to developing cracks, and/or to peel and humidity."
Treadway et al. disclose coating compositions comprising an organic solvent and at least two other components dissolved therein. One component is a nitrogen-containing, partially hydrolyzed condensate derived from an aminoalkylalkoxysilane precursor. The second component is an epoxy-containing, partially hydrolyzed compound derived by partial hydrolysis from an epoxyalkylalkoxysilane precursor. The first component may be an imine which reportedly extends the working life of the coating composition. It is believed that the imine is in equilibrium with free amine and that this equilibrium inhibits the reaction between the amine and the epoxy functionality of the second component because of the limited availability of free amine. However, when the coating composition is heated during use, the equilibrium is disturbed, more amine is generated, and the amine and epoxy functionalities cross-link.
The Treadway et al. patent is discussed in U.S. Pat. No. 4,800,122 "Siloxane-Based Tintable Coating," issued Jan. 24, 1989 to R. Sallavanti et al., which states that "considerable room for improvement remains in terms of abrasion resistance and tintability." Sallavanti et al. employ a non-silane-based aliphatic polyamine which is reacted with a ketone or an aldehyde, presumably to form an imine. This material is mixed with a partially hydrolyzed epoxysilane which regenerates the polyamine. The regenerated polyamine ultimately reacts with the epoxy groups of the epoxy silane to form a cross-linked polymeric network.
U.S. Pat. No. 4,127,697 "Abrasion-Resistant Lenses and Process of Making," issued Nov. 28, 1978 to B. Laurin, discloses the use of a tie-coat to improve adhesion of an abrasion resistant coating to a plastic substrate.
Preferably, coating compositions for plastic ophthalmic lenses should offer features in addition to abrasion resistance. For example, the compositions should be storage stable (i.e., have an extended shelf life). Many previously known compositions are "two part" formulations that require mixing two materials (which then react) to provide the ultimate composition. However, two part formulations react rapidly after mixing and must be used quickly. This may be inconvenient, wasteful and expensive. A user of the coating composition must maintain separate inventories of the two materials and can only mix them shortly before use. To effectively use the coating composition may require preparing a significantly larger volume than is actually needed. Since the compositions rapidly become unusable, expensive materials may be wasted.
The Koda et al. patent attempts to address this problem and effectively provides a "one part" system that reportedly remains stable for up to 30 days after mixing. However, Treadway et al. observed that stability was achieved by high dilution which requires several coatings to obtain the desired film thickness.
Coating compositions for ophthalmic devices must also be readily tintable to provide lenses of various shades so as to satisfy consumer demands. Sallavanti et al. characterize the Treadway et al. patent as leaving considerable room for improvement in terms of tintability.